Level 3 Sending spell can cross planes with only minor chance of failure, but sender needs to be already familiar with sendee. There are work-arounds and Zariel has greater powers at her disposal, so pretty sure she could communicate with Anders direct.
Thing is, she's not a demigod, so can't create simultaneous avatars. Her agenda is packed full therefore need to prioritize actions or delegate to lieutenants. If Karlach was as important as we're led to believe, Zariel could have made it a priority that day.
Dead member in the toll house talks about pact power, so probably warlock. With direct connection, that character could've been the lynchpin to bring the band together, and thus Speak with dead spilled the beans.
